topography topics

The Topography Red
Zone

BY LORETTA B. SZCZOTKA, OD, MS
March 2000

A common misconception when interpreting a corneal topography map is to immediately assume that any red zone is a high point on the cornea. Many of us are guilty of making this
assumption, especially when trying to explain a map to a keratoconic patient by pointing to a red "peak" or a blue "valley." However, the only time this conclusion is valid is when viewing an elevation map. Occasionally, this interpretation may be valid for a curvature map, but there are many exceptions.

Distinctive Characteristics

Most elevation maps depict relative height differences from a computer-generated reference sphere. Red, warm colors denote higher elevation values and blue, cool colors denote areas of the cornea that are lower than the specified reference sphere. Confusion among novice corneal topography users arises because the colors are identical to those used in curvature displays. However, when viewing elevation maps, they represent height differences, not curvatures.

Curvature maps display data very differently than elevation maps. On curvature maps, both steep (red) and flat (blue) areas may be high or low in relation to a reference sphere. The difference depends on the geographic position of the steep zone. For example, steep, red curvatures will be high when centrally located, but will result in a relative low point when located in the corneal periphery.

In Figure 1, axial and tangential keratoconus maps are displayed. Centrally, the red steep zone will be close to the relative "peak" of the cornea, but in the mid and far periphery, the steep area will actually pool fluorescein underneath a spherical RGP lens because it's lower than the reference sphere. Figure 2 illustrates the dramatic difference in the interpretation of the red zone on a post-penetrating keratoplasty patient. Note the upper left axial curvature map, which displays significant peripheral corneal steepening relative to the center of the map at the 4 and 10 o'clock zones. On the lower left elevation map, these same areas are depicted as relative low zones on the corneal surface.

Clinically, when placing a spherical RGP lens on the eye in this example, the high and low areas are confirmed: fluorescein will pool at the 4 and 10 o'clock positions, and the cornea will displace fluorescein at the 1 and 7 o'clock positions.

Remember, steep doesn't mean high, and red only means high on elevation maps!